The mutation of guanine to thymine, leading to substitution of glutamate to aspartate at position 298 in exon 7 of the endothelial nitric oxide synthase (eNOS) gene has been associated with coronary artery spasm and myocardial infarction. Studies to determine the effects of this polymorphism on physiological nitric oxide (NO) dependent process of platelet aggregation and arterial vasodilatation were undertaken. Platelet aggregation studies indicated increased collagen-induced platelet aggregation in individuals homozygous for the variant allele. Stimulation-dependent NO release from activated platelets was greater in subjects who are homozygous for guanine (GG) as compared to subjects carrying thymine in one allele (GT) and subjects homozygous for thymine (TT). Superoxide released from activated platelets was increased in TT homozygotes as compared to GG homozygotes. To evaluate the vascular endothelial effects of this polymorphism, forearm vascular responses were determined using venous occlusion plethysmography. There were no differences among the three groups (GG, GT, TT) in endothelium-dependent or endothelium independent vasodilatation. These findings suggest that the association of this polymorphism with cardiovascular events may be due in part to enhanced platelet reactivity. Study of the (eNOS)polymorphism in the promoter region (thymine to cytosine at position 786) in combination with the GG,GT, and TT alleles has been undertaken. This promoter variant has been associated with decreased eNOS activity in in vitro study. Forearm vascular relaxation was determined as above. Preliminary data indicate decreased acetylcholine mediated vascular relaxation (endothelium dependent) in subjects who carry the promoter variant and a greater decrease in those who carry both the promoter variant and the TT variant. These findings suggest that defining genetic variants for a relatively small number of effector genes can explain substantial variability in forearm vascular responsiveness. Future studies will evaluate combined polymorphisms of genetic variants that have been associated with cardiovascular disease. These will include variants of angiotensin converting enzyme, the angiotensin receptor, and apolipoprotein E.